"Dave Plowman (News)" wrote in message
...
In article ,
Arfa Daily wrote:
Ahh, grasshopper, to modify the 12v rail first you must find the
voltage reference for the 5 v or 3.3 v rail, your journey will take
you into the realms of voltage dividers and the TL431.

Ain't got one of those.


Yes it has, Dave. With that particular supervisory IC, it's behind the
reference and feedback pins, and not an external discrete device, as I
previously explained ...

Right - having read the spec more carefully it does indeed state it
contains two of these. So please accept my apologies.

If you're not understanding the principles of
how these things regulate, then I don't think you are going to get too
far with this project.

Well, if I were an expert on them I'd hardly need to ask here.
It's not for some life support mechanism, you know.

--
*Bigamy is having one wife too many - monogamy is the same

Dave Plowman London SW
To e-mail, change noise into sound.

I didn't intend any offence, Dave - probably could have picked my words
better. All I was trying to imply, is that switch mode power supplies are a
bit of a black art, as you have discovered, and unless you are working with
them all the time, and thus understand the subtleties of some of the
circuitry and regulation techniques which they employ, then this project is
quite likely to tie you up in knots. I must admit that I didn't look too
closely at the application schematic to see how exactly they went about
regulating the thing, but when I saw the two undedicated adjustable zeners
in there, I assumed that they were probably making use of at least one of
them in the primary feedback loop. It is a common technique to use an
(usually external) adjustable zener to achieve main rail set voltage and
regulation.

Franc is probably right in this case with his proposal of altering the
resistors on the IN pin. You may well find that when you put the pot in, you
actually had the desired effect of increasing or decreasing the output
voltage, but then fell foul of the under / over voltage lockout feature,
which I think I said that you might have to disable. If you put a digital
meter on the output, and watch very closely, you might see the voltage start
to alter up and down, before it trips out.

Arfa